Techniques for detecting positions of an activated main spindle in a NC machine tool such as a machining center, for detecting positions of an activated feeding system such as a table or the like and measuring devices employed for those are known. Measurement of positions in these cases is necessary, e.g., for correcting thermal deformation or the like occurring in the machine tool during machining process. Measurement is also made for checking figures, abrasion or the like. Objects for measurement in these cases are tool positions of the main spindle, tool lengths, tool figures or a feeding system such as a table.
Measurement is carried out in a machine tool during operation mainly concerned with whether there is or is not thermal deformation, abrasion, breaking, etc. generated by the machining action performed in the machine tool. Thermal deformation among these is specifically concerned with machining precision because it has a direct effect on the relative position between a tool and a work by generating positional deviation. In many cases main spindles specifically suffer from thermal deformation due to high speed operation in recent years. Also in the feeding system relative positional deviation is generated between itself and the main spindle under the thermal effect of a ball screw. Further, deviation of relative position between the main spindle and the table occurs during machining because of combination of various factors such as deformation generated by attachment of a tool, abrasion of the tool or the like.
For these reasons, measurement has been made for related portions and positional correction of tools or the like has been made according to the result of measurement. A typical method for measuring position is one by use of a touch sensor. This method is such that a touch probe device is attached to a main spindle, position is measured by causing the touch probe device to make contact with a work through relative movement between the work and an spindle, measurement of position is performed and final positioning is made by NC control. This method is applied in many cases and a technique for correcting position as an example for correction of thermal deformation is known in which correction of thermal displacement based on the measured temperature and automatic centering by making zero correction of varied coordinate with a touch sensor attached to the main spindle are combined so as to complement each other at determined times (see e.g. Japanese Patent Laid-open H6-304846).
A technique for correcting totally thermal displacement both of a feed screw in a machine tool and a work, that is, a technique in which thermal displacement of a ball screw is compared with the absolute standard by an invar material with little thermal expansion and a touch sensor is used for making correction based on the comparison, so that correction of thermal displacement in a work is made through control based on the reference temperature is known (see e.g. Japanese Patent Laid-open 2006-212765). The structure of a touch probe for three dimensional measurement concerned with these and a technique for extracting detection signal are also known (see e.g. Japanese Patent Laid-open H6-185911).
Recently it has also become known to use noncontact measuring devices. As an example for this, a technique for detecting position is known in which measuring means is caused to make approach to laser beam and the position at the moment when the laser beam is interrupted by the measuring device is measured. Relating to this, a technique is known in which a measuring device by use of laser beam is mounted on a turn table capable of indexing by 90 degrees on the table of a machine tool and a tool is made to approach the laser beam so that the position of axis line is measured in the situation where the position of axis line is changed by 90 degrees (see e.g. Japanese Patent Laid-open H10-138097).
This method corresponds to a technique for obtaining the position by calculating the difference from the reference value obtained by preliminary measurement. Further, as a technique which allows the top end position of the tool during rotation in a machine tool, the diameter of the tool, the length of the tool or the like to be measured quickly, a technique for measurement is known in which an optical measuring device emitting laser beam is placed at a location on the table where no obstruction occurs for machining operation and then measurement is performed (see e.g. Japanese Patent Laid-open H11-138392). As a technique concerned specifically with correction of thermal displacement, also a technique is known in which variation is obtained by use of the temperature measured with a thermometer and coefficients for correction are obtained so as to perform correction (see e.g. Japanese Patent Laid-open H5-116053). This is not a technique in which numerical values are obtained physically with a measuring device provided on the body of a machine tool but is an example of a technique for postulating the position based on the measurement of temperature.
Specifically as techniques for measuring position by use of laser beam, techniques are already known in which approach to the laser beam is made at a right angle as mentioned above. However, those techniques in the present state are such that measurement is made for a spindle moving in an axial direction, while, for coping with measurement by interrupting the same laser beam with a thing moving in the direction of another axis, the direction of the laser beam is changed by turning the table by 90 degrees as mentioned above or another laser beam is provided for another axis.
Thus, respective measurement operations have suffered from inefficiency, which has caused efficiency of machining to be lowered. Further, in case of using a touch sensor the effect of attached dirt can not be neglected and so it has been desired to secure a circumstance in which measurement is allowed to be performed. Especially the thermal displacement of a feeding system due to the thermal displacement of the main spindle and the ball screw is a factor which can not be neglected. The present invention is devised in order to solve the above mentioned conventional problems and attains the following objects.